Eliminate code duplication in vn_fullpath1() around the cache lookups

Eliminate code duplication in vn_fullpath1() around the cache lookups
and calls to vn_vptocnp() by moving more of the common code to
vn_vptocnp(). Rename vn_vptocnp() to vn_vptocnp_locked() to signify that
cache is locked around the call.

Do not track buffer position by both the pointer and offset, use only
buflen to record the start of the free space.

Export vn_vptocnp() for external consumers as a wrapper around
vn_vptocnp_locked() that locks the cache and handles hold counts.

Tested by:	pho